Radio frequency identification (RFID) systems typically use one or more reader antennae to send radio frequency (RF) signals to items comprising RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the RF signals from a reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from a reader antenna when the tags are within the detection range of the reader antenna.
The detection range of the RFID systems is typically limited by signal strength over short ranges, for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units may be moved past a group of tagged items in order to detect all the tagged items, particularly where the tagged items are stored in a space significantly greater than the detection range of a stationary or fixed single reader antenna. Alternately, a large reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of the radiated power beyond allowable limits. Furthermore, these reader antennae are often located in stores or other locations where space is at a premium and it is expensive and inconvenient to use such large reader antennae. Alternatively, multiple small antennae may be used. However, such a configuration may be awkward to set up when space is at a premium and wiring is preferred or required to be hidden.
Current RFID reader antennae are designed to maintain a maximum read range between the antenna and associated tags, without violating FCC regulations regarding radiated emissions. When tagged items are stacked, the read range of an antenna can be impeded due to “masking” of the stacked, tagged items. As a result, the masking limits the number of tags that an antenna may read at a given time, and consequently affects the number of products that may be read.
Resonant reader antenna systems are currently utilized in RFID applications, where numerous reader antennae are connected to a single reader. Each reader antenna may have its own tuning circuit that is used to match to the systems characteristic impedance. However, multiple reader antennae (or components thereof) cannot be individually controlled when they are connected by a single transmission cable to a reader unit.